Lens systems are used to more clearly view surfaces of objects by magnifying views of the surfaces. The systems may include one or more lenses that change the focus of light by refracting the light. The lenses may converge or diverge the light in order to magnify views of the surfaces. For example, microscopes may be used to magnify very small objects and allow a human operator to view the small objects that would otherwise be impossible for the operator to view.
Equipment such as turbines may have very small objects that need to be viewed in order to check on the status and/or sizes of the objects. For example, geometry dimensions of cooling holes in turbine or airfoil blades are important to ensure cooling efficiency of the airfoils. The ability to measure dimensional parameters of the cooling holes can help improve the manufacturing process of the cooling holes and airfoil blades.
Currently, there is no known reliable measurement system to verify the dimensions of cooling holes in order to ensure that the dimensions are within desired ranges or sizes. In order to obtain high resolution images of the cooling holes, short focal length objective lenses are needed to achieve sufficient magnification. But, the working distances of known short focal length objective lenses are limited. In order to measure the cooling holes on larger airfoil blades and nozzles, a relatively large standoff distance (e.g., a distance between the object being viewed and the lens) may be needed due to obstruction of other components of the turbines that include the airfoil blades and nozzles. For example, at least a 70 millimeter or other standoff distance may be required. However, in order to maintain the resolution required in the image formed by the lens, a high numerical aperture (e.g., the sine of the cone angle of light collected) also may be needed. That is, the ability of a lens to produce an image of a particular size feature is inherently limited by the cone angle of rays of the light diffracted by a given size feature. A wider collection cone angle provides information to resolve a smaller size feature. This effect, diffraction limit, is well known in the field of optics. There are no currently known objective lenses having sufficiently long working distances and sufficiently high numerical aperture that can provide the physical standoff, the image resolution and magnification needed to view the geometric dimensions of cooling holes in airfoil blades.